Our studies are being directed toward a complete understanding of the cells involved in the mucosal immune response. This clearly encompasses antigen processing and presentation by accessory cells (macrophages), as well as positive and negative regulation by T lymphocytes. Our work and others clearly suggests the existence of a common mucosal immune system where antigen either ingested or inhaled sensitizes gut- or bronchial-associated lymphoid tissue (GALT or BALT) and ultimately results in IgA responses in secretory areas including salivary and mammary glands. GALT cells are continually influenced by environmental antigens such as lipopolysaccharide (LPS) from the Gram negative gut flora and the inductive events of antigen stimulation of GALT must consider this influence. Our results clearly indicate that LPS affects T lymphocytes in GALT which can then suppress GALT responses to certain T dependent antigens. LPS nonresponsive (C3H/HeJ) mice do not harbor this population of cells and thus, manifest elevated IgA responses. This mouse strain offers a useful model for evaluation of oral adjuvants which can enhance musosal immune responses. Since bacterial vaccines including Streptococcus mutans contain T dependent antigens, it is imperative that careful selection of animal models be made in order to most effectively evaluate the dose and form of antigen which will lead IgA responses in the appropriate external secretion.